MINOR: global: define diagnostic mode of execution

MINOR: global: define diagnostic mode of execution

Define MODE_DIAG which is used to run haproxy in diagnostic mode. This
mode is used to output extra warnings about possible configuration
blunder or sub-optimal usage. It can be activated with argument '-dD'.

A new output function ha_diag_warning is implemented reserved for
diagnostic output. It serves to standardize the format of diagnostic
messages.

A macro HA_DIAG_WARN_COND is also available to automatically check if
diagnostic mode is on before executing the diagnostic check.